Which sentence does not contain any errors in comma usage?
a. well i'm not positive that's healthy.
b. while you are waiting, for the doctor, you should consider reading a magazine.
c. running before dawn billy got a good workout.
d. in the desk drawer, there is a spare key to the house.

The greatest gift the sumerians gave the world was the invention of writing. the sumerians were a wealthy people. they needed some way to keep track of what they owned. they began drawing pictures. they used a reed as a pen. they drew on soft pieces of clay. the soft clay was then dried in the sun. the tablet became a permanent record. later, the sumerian drawings changed into wedge-shaped symbols. this kind of writing is called cuneiform. by putting symbols together, the sumerians could write entire sentences.